1. Field of the Invention
The present invention relates to valve assemblies of the rotary type in which the stem is provided with a mechanically loaded back-seat with externally variable stem seating force so as to prevent fugitive emissions by isolating the stem packing from the fluid in the valve body cavity and thus prolonging the useful life of the stem packing.
2. Brief Description of Prior Art
Valves are used in large numbers in the process industry handling fluids of all kinds and they are the major contributor to overall fugitive emissions in the industry. Fugitive emission is referred to any fluid, hazardous or otherwise, leaking through any part of a closed piping system. Leakage around the valve stem through the stem packing is a significant cause of fugitive emissions. The Clean Air Act establishes stringent guidelines for control of fugitive emissions.
A common method of controlling leakage around the valve stem is to provide a stem seal that includes the use of a stuffing box that is packed with material such as graphite or fluoropolymers. The bolted gland of the stuffing box exerts compressive force on the packing material which in turn exerts radial thrust around the stem periphery to seal the stem passageway. When the stem packing is newly installed, the leakage around the stem is minimal. But when the valve is used, the stem reciprocates and/or rotates inside the packing material which eventually wears out and the radial thrust around the stem periphery relaxes. The decreased radial thrust reduces the sealing effectiveness around the stem, thereby permitting fluid in the valve body cavity to leak along the stem passageway. In order to reduce fluid leakage, the stuffing box packing is tightened axially in order to increase the radial sealing thrust around the stem. The stuffing box packing is also replaced periodically with new packing material. Another method to reduce fugitive emissions around the stem is to put stem packing under "live loading" with springs which provides a constant compressive force on the stem packing.
The above method of stem packing only controls fugitive emission and does not prevent it. Fugitive emission around the stem can be prevented only by isolating the stem packing from the fluid in the valve body cavity.
In valves that are in use today, only non-rotary valves with reciprocating stems are provided withback-seats that can be loaded with externally variable stem seating force. So far it has been found very difficult and elusive to provide back-seats on rotary valve stems that can be loaded with externally variable stem seating force that automatically adjusts to wear of the seating surfaces. Some current rotary valves like plug and ball valves are provided with stems that have back-seats which are either fluid pressure energized or loaded with springs which provide a constant seating force and do not automatically self-adjust for wear.
Therefore, there exists a need today in which rotary valves could be provided with back-seats for the stem whereby the back-seating force automatically self-adjusts for wear of the seating surfaces independent of fluid pressure.
Prior art Blevans U.S. Pat. No. 2,719,022 of Sep. 27, 1955 does show a rotary valve in which the stem passageway is sealed from the valve body cavity. But this sealing is not effective because of stem back-seating and the sealing force is not automatically externally variable. The sealing of the stem passageway is caused by plastic packing injected under pressure around the outer periphery of a stem sealing ring located in the valve body around the stem in the stem passageway. This method is not foolproof by any means. The method is not a positive way to seal the stem passageway from the valve body cavity. The sealing integrity around the stem will be compromised, if there is a drop in the pressure of the injected plastic packing material and this is bound to happen sooner or later. Secondly, there is no automatic self-adjustment for wear of the seating surfaces. Also the plastic packing will have to be injected under such a high pressure that resistance to stem rotation will be encountered. Blevans rotary valve is extremely complex and expensive for manufacture and sealing around the stem in the stem passageway cannot be assured.
Prior art Gray U.S. Pat. No. 2,124,334 shows a gate valve wherein the stem is back-seated in both opened and closed positions of the valve. However, Gray U.S. Pat. No. 2,124,334 applies to a reciprocating gate valve wherein the closure member reciprocates between open and closed positions of the valve. Also during Gray's times there was no pressing need for the prevention of fugitive emissions. The instant patent application refers to rotary valves only in which the closure member rotates during open and closed positions of the valve.